Over many years, glass manufacturing industry has aimed at manufacturing homogeneous glass products containing no contaminants, bubbles, and the like through melting at high efficiency, and supplying glass products that meet demands. The glass manufacturing industry has attempted to attain an object of manufacturing homogeneous glass in various glass products, and numerous inventions have been made to attain the object. Although factors causing inhomogeneous glass such as stones, cords, and precipitation of heterogeneous crystals in glass can be eliminated, an object of removing bubbles completely remains unattained. The removal of bubbles generally employs a method involving: addition of fining agents in a raw material composition; expansion and elevation of bubbles of very small diameters through a redox reaction of fining agents; and deaeration of the bubbles from molten glass.
As shown in FIG. 11, in general, a glass melting furnace for performing continuous production of glass is often constituted by a melting tank 10 for melting glass by heating, and a fining chamber 30 for deaerating bubbles from molten glass G after the melting. In such a continuous production method, a glass raw material B including various glass raw material powder and glass cullet is homogeneously mixed in advance, and the glass raw material B is continuously charged from an inlet 11 in the melting tank 10 of the melting furnace formed of a refractory material R. Inside of the melting tank 10 is heated to high temperatures by a burner 31 and an electrode 12 for a vitrification reaction of the glass raw material B. Then, oxygen bubbles and the like are generated through a redox reaction of fining agents added into the glass raw material B in the fining chamber 30 connected to the melting tank 10, to expand bubbles having very small diameters of a carbon dioxide gas and the like generated during the vitrification reaction and to elevate the bubbles to a surface of the molten glass G for fining. Thereafter, the molten glass G is homogenized with a stirrer 51 provided in a feeder 50, formed into a predetermined shape in a forming part, and annealed into a glass product.
When continuous production is not performed, the production is performed in a so-called batch production furnace employing a crucible of quartz, alumina, or the like, a crucible of another refractory material, or a vessel (pot) formed of a heat resistant metal containing platinum as shown in FIG. 12. In this case, molten glass is homogenized through processes of melting and fining in one tank. To be specific, glass raw materials are charged into a refractory vessel 70 protected by a refractory material R and provided inside the refractory material R. The glass raw materials in the heat resistant vessel 70 are indirectly heated by a heating element 41. A series of processes including fining is performed in the same tank for the molten glass G produced through a vitrification reaction. The molten glass G is homogenized with a stirrer 51, and is allowed to flow out from an outlet 60 provided at a bottom of a melting tank. The molten glass G is formed into a predetermined shape and annealed into a glass product.
In order to attain an object of more assuredly fining bubbles, which are defects generated in the molten glass, Patent Document 1 describes a method of deaerating bubbles of very small diameters by introducing a gas such as air, oxygen, or argon as bubbles into the molten glass through a bubbling nozzle provided in the above-described fining chamber. Further, Patent Document 2 discloses a technique referred to as vacuum degassing for deaerating bubbles in the molten glass for homogenization by intentionally adjusting a pressure of a glass melting atmosphere to a pressure lower than an atmospheric pressure.
Further, Patent Document 3 describes use of a helium atmosphere for remelting a glass cullet to prevent reboiling of bubbles. However, Patent Document 3 includes no description regarding an effective helium content in glass or the like, and lacks detail. Patent Document 4 describes the use of helium in a dry gas to be bubbled. However, Patent Document 4 includes no description regarding specific modes of a bubbling device or a helium content in glass, and thus cannot be applied to a glass manufacturing apparatus allowing mass production. Patent Document 5 also describes the use of helium bubbling. However, Patent Document 5 includes no specific description regarding a device for bubbling helium or a helium content in glass, and thus cannot be applied to a glass manufacturing apparatus allowing mass production.
[Patent Document 1] Japanese Utility Model Registration No. 2583790
[Patent Document 2] JP-A 2000-239023
[Patent Document 3] JP-A 06-329422
[Patent Document 4] JP-A 07-172862
[Patent Document 5] U.S. Pat. No. 3,622,296